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Teach the DAG combiner to turn chains of FADDs (x+x+x+x+...) into FMU…
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…Ls by constants. This is only enabled in unsafe FP math mode, since it does not preserve rounding effects for all such constants.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162956 91177308-0d34-0410-b5e6-96231b3b80d8
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@resistor
resistor committed Aug 30, 2012
1 parent fafa283 commit 43da6c7
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Showing 2 changed files with 159 additions and 1 deletion.
123 changes: 122 additions & 1 deletion lib/CodeGen/SelectionDAG/DAGCombiner.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -5681,6 +5681,127 @@ SDValue DAGCombiner::visitFADD(SDNode *N) {
DAG.getNode(ISD::FADD, N->getDebugLoc(), VT,
N0.getOperand(1), N1));

// In unsafe math mode, we can fold chains of FADD's of the same value
// into multiplications. This transform is not safe in general because
// we are reducing the number of rounding steps.
if (DAG.getTarget().Options.UnsafeFPMath &&
TLI.isOperationLegalOrCustom(ISD::FMUL, VT) &&
!N0CFP && !N1CFP) {
if (N0.getOpcode() == ISD::FMUL) {
ConstantFPSDNode *CFP00 = dyn_cast<ConstantFPSDNode>(N0.getOperand(0));
ConstantFPSDNode *CFP01 = dyn_cast<ConstantFPSDNode>(N0.getOperand(1));

// (fadd (fmul c, x), x) -> (fmul c+1, x)
if (CFP00 && !CFP01 && N0.getOperand(1) == N1) {
SDValue NewCFP = DAG.getNode(ISD::FADD, N->getDebugLoc(), VT,
SDValue(CFP00, 0),
DAG.getConstantFP(1.0, VT));
return DAG.getNode(ISD::FMUL, N->getDebugLoc(), VT,
N1, NewCFP);
}

// (fadd (fmul x, c), x) -> (fmul c+1, x)
if (CFP01 && !CFP00 && N0.getOperand(0) == N1) {
SDValue NewCFP = DAG.getNode(ISD::FADD, N->getDebugLoc(), VT,
SDValue(CFP01, 0),
DAG.getConstantFP(1.0, VT));
return DAG.getNode(ISD::FMUL, N->getDebugLoc(), VT,
N1, NewCFP);
}

// (fadd (fadd x, x), x) -> (fmul 3.0, x)
if (!CFP00 && !CFP01 && N0.getOperand(0) == N0.getOperand(1) &&
N0.getOperand(0) == N1) {
return DAG.getNode(ISD::FMUL, N->getDebugLoc(), VT,
N1, DAG.getConstantFP(3.0, VT));
}

// (fadd (fmul c, x), (fadd x, x)) -> (fmul c+2, x)
if (CFP00 && !CFP01 && N1.getOpcode() == ISD::FADD &&
N1.getOperand(0) == N1.getOperand(1) &&
N0.getOperand(1) == N1.getOperand(0)) {
SDValue NewCFP = DAG.getNode(ISD::FADD, N->getDebugLoc(), VT,
SDValue(CFP00, 0),
DAG.getConstantFP(2.0, VT));
return DAG.getNode(ISD::FMUL, N->getDebugLoc(), VT,
N0.getOperand(1), NewCFP);
}

// (fadd (fmul x, c), (fadd x, x)) -> (fmul c+2, x)
if (CFP01 && !CFP00 && N1.getOpcode() == ISD::FADD &&
N1.getOperand(0) == N1.getOperand(1) &&
N0.getOperand(0) == N1.getOperand(0)) {
SDValue NewCFP = DAG.getNode(ISD::FADD, N->getDebugLoc(), VT,
SDValue(CFP01, 0),
DAG.getConstantFP(2.0, VT));
return DAG.getNode(ISD::FMUL, N->getDebugLoc(), VT,
N0.getOperand(0), NewCFP);
}
}

if (N1.getOpcode() == ISD::FMUL) {
ConstantFPSDNode *CFP10 = dyn_cast<ConstantFPSDNode>(N1.getOperand(0));
ConstantFPSDNode *CFP11 = dyn_cast<ConstantFPSDNode>(N1.getOperand(1));

// (fadd x, (fmul c, x)) -> (fmul c+1, x)
if (CFP10 && !CFP11 && N1.getOperand(1) == N0) {
SDValue NewCFP = DAG.getNode(ISD::FADD, N->getDebugLoc(), VT,
SDValue(CFP10, 0),
DAG.getConstantFP(1.0, VT));
return DAG.getNode(ISD::FMUL, N->getDebugLoc(), VT,
N0, NewCFP);
}

// (fadd x, (fmul x, c)) -> (fmul c+1, x)
if (CFP11 && !CFP10 && N1.getOperand(0) == N0) {
SDValue NewCFP = DAG.getNode(ISD::FADD, N->getDebugLoc(), VT,
SDValue(CFP11, 0),
DAG.getConstantFP(1.0, VT));
return DAG.getNode(ISD::FMUL, N->getDebugLoc(), VT,
N0, NewCFP);
}

// (fadd x, (fadd x, x)) -> (fmul 3.0, x)
if (!CFP10 && !CFP11 && N1.getOperand(0) == N1.getOperand(1) &&
N1.getOperand(0) == N0) {
return DAG.getNode(ISD::FMUL, N->getDebugLoc(), VT,
N0, DAG.getConstantFP(3.0, VT));
}

// (fadd (fadd x, x), (fmul c, x)) -> (fmul c+2, x)
if (CFP10 && !CFP11 && N1.getOpcode() == ISD::FADD &&
N1.getOperand(0) == N1.getOperand(1) &&
N0.getOperand(1) == N1.getOperand(0)) {
SDValue NewCFP = DAG.getNode(ISD::FADD, N->getDebugLoc(), VT,
SDValue(CFP10, 0),
DAG.getConstantFP(2.0, VT));
return DAG.getNode(ISD::FMUL, N->getDebugLoc(), VT,
N0.getOperand(1), NewCFP);
}

// (fadd (fadd x, x), (fmul x, c)) -> (fmul c+2, x)
if (CFP11 && !CFP10 && N1.getOpcode() == ISD::FADD &&
N1.getOperand(0) == N1.getOperand(1) &&
N0.getOperand(0) == N1.getOperand(0)) {
SDValue NewCFP = DAG.getNode(ISD::FADD, N->getDebugLoc(), VT,
SDValue(CFP11, 0),
DAG.getConstantFP(2.0, VT));
return DAG.getNode(ISD::FMUL, N->getDebugLoc(), VT,
N0.getOperand(0), NewCFP);
}
}

// (fadd (fadd x, x), (fadd x, x)) -> (fmul 4.0, x)
if (N0.getOpcode() == ISD::FADD && N1.getOpcode() == ISD::FADD &&
N0.getOperand(0) == N0.getOperand(1) &&
N1.getOperand(0) == N1.getOperand(1) &&
N0.getOperand(0) == N1.getOperand(0)) {
return DAG.getNode(ISD::FMUL, N->getDebugLoc(), VT,
N0.getOperand(0),
DAG.getConstantFP(4.0, VT));
}
}

// FADD -> FMA combines:
if ((DAG.getTarget().Options.AllowFPOpFusion == FPOpFusion::Fast ||
DAG.getTarget().Options.UnsafeFPMath) &&
Expand All @@ -5692,7 +5813,7 @@ SDValue DAGCombiner::visitFADD(SDNode *N) {
return DAG.getNode(ISD::FMA, N->getDebugLoc(), VT,
N0.getOperand(0), N0.getOperand(1), N1);
}

// fold (fadd x, (fmul y, z)) -> (fma x, y, z)
// Note: Commutes FADD operands.
if (N1.getOpcode() == ISD::FMUL && N1->hasOneUse()) {
Expand Down
37 changes: 37 additions & 0 deletions test/CodeGen/X86/fp-fast.ll
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Original file line number Diff line number Diff line change
@@ -0,0 +1,37 @@
; RUN: llc -march=x86-64 -mtriple=x86_64-apple-darwin -enable-unsafe-fp-math < %s | FileCheck %s

; CHECK: test1
define float @test1(float %a) {
; CHECK-NOT: vaddss
; CHECK: vmulss
; CHECK-NOT: vaddss
; CHECK: ret
%t1 = fadd float %a, %a
%r = fadd float %t1, %t1
ret float %r
}

; CHECK: test2
define float @test2(float %a) {
; CHECK-NOT: vaddss
; CHECK: vmulss
; CHECK-NOT: vaddss
; CHECK: ret
%t1 = fmul float 4.0, %a
%t2 = fadd float %a, %a
%r = fadd float %t1, %t2
ret float %r
}

; CHECK: test3
define float @test3(float %a) {
; CHECK-NOT: vaddss
; CHECK: vxorps
; CHECK-NOT: vaddss
; CHECK: ret
%t1 = fmul float 2.0, %a
%t2 = fadd float %a, %a
%r = fsub float %t1, %t2
ret float %r
}

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